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Remediation of Metal-ContaminatedSoil by an Integrated Soil Washing-Electrolysis ProcessShih-Hsien Chang a , Kai-Sung Wang a , Chung-Yih Kuo a , Chih-YuanChang a & Ching-Tung Chou aa Department of Public Health , Chung-Shan Medical University ,Taichung, Taiwan, ROCPublished online: 18 Jan 2007.

To cite this article: Shih-Hsien Chang , Kai-Sung Wang , Chung-Yih Kuo , Chih-Yuan Chang & Ching-Tung Chou (2005) Remediation of Metal-Contaminated Soil by an Integrated Soil Washing-ElectrolysisProcess, Soil and Sediment Contamination: An International Journal, 14:6, 559-569

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Soil & Sediment Contamination, 14:559–569, 2005Copyright © Taylor & Francis Inc.ISSN: 1532-0383 print / 1549-7887 onlineDOI: 10.1080/15320380500263758

Remediation of Metal-Contaminated Soil by anIntegrated Soil Washing-Electrolysis Process

SHIH-HSIEN CHANG, KAI-SUNG WANG, CHUNG-YIH KUO,CHIH-YUAN CHANG, CHING-TUNG CHOU

Department of Public Health, Chung-Shan Medical University, Taichung,Taiwan, ROC

Chelating agents such as EDTA and DTPA are often used to remove metals from soil.However, their toxicity, bio-recalcitrance, and problems with recovery of heavy metaland chelating agents severely limit their applications. A biodegradable chelating agent,LED3A, and two surfactants, SDS and Triton X 100, were evaluated as potential alter-natives for remediation of metal-contaminated soil.

LED3A alone only removed 40% of cadmium the addition of surfactant signif-icantly enhanced its cadmium removal capacity up to 80% for a wide range of pH(5 to 11). The enhancement increased with both surfactant concentrations and LED3Aconcentrations. Because LED3A had a much higher removal capacity for copper, thesynergistic effect of surfactant-LED3A mixture was less obvious. Sequential extractionanalysis indicated that the LED3A not only removed copper from carbonate and Fe-Mnoxide fraction, but also from organic fractions. A three-dimension electrolysis reactorcould effectively recover both metals and LED3A-SDS within thirty minutes. The com-bined soil washing by LED3A-surfactants and electrolysis provides a potential approachfor remediation of copper- and cadmium-contaminated soils.

Keywords Soil washing, sequential extraction, LED3A, electrolysis.

1. Introduction

The presence of heavy metals in soil poses risks to health and the environment. Soil wash-ing is an inexpensive and practical technique for metal removal. The use of diluted acidscan dissolve soil matrix and damage soil physiochemical and biological properties (Baronaet al., 2001). Chelating agents such as ethylene diamine tetraacetic acid (EDTA) and di-ethylene trinitrilo pentaacetic acid (DTPA) have proven effective in removing metals fromsoils (Peters, 1999; Hong et al., 2002). Their biotoxicity, biorecalcitrance, and problemswith recovery of heavy metals and extracting agents from supernatant severely limit theirapplications (Hong et al., 2002).

Surfactants are amphiphilic molecules with a hydrophilic head group and a hydropho-bic tail group. Based on their hydrophilic head groups, surfactants can be divided into threecategories: anionic, cationic, and nonionic (Huang et al., 1997). Surfactants at low con-centrations exist as monomers and adsorb onto surfaces and interfaces. When surfactant

This study was supported by the National Science Council of Taiwan under contract NSC91-2211-E-040-001.

Address correspondence to S.-H. Chang, Department of Public Health, Chung-Shan MedicalUniversity, Taichung 402, Taiwan, ROC. E-mail: shchang@csmu.edu.tw

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concentrations increase above a certain level (i.e. critical micelle concentration, CMC), themonomers aggregate to form micelles, which have a hydrophobic interior and a hydrophilicexterior (Haigh, 1996). Although anionic surfactants could increase heavy metal removalfrom soil, they are much less effective compared to chelating agents (Nivas et al., 1996;Doong et al., 1998; Gadelle et al., 2001). The information on interactions of chelatingagents and surfactants on soil metal removal is scarce.

Sequential extraction analysis can help identify the geochemical fractions of metalsin soil (Tessier et al., 1979). Basically, the more stable the metal binding is with the soil,the stronger extractants are required. Heavy metals in exchangeable and carbonate formsare easily solubilized by acids. In contrast, heavy metals bound to organic and crystallinelattice are difficult to extract (Gleyzes et al., 2002).

LED3A (Lauroyl-ED3ANa2, Dow company) is structurally similar to EDTA. It hypoth-esizes that LED3A should have a similar metal extractive capacity as that of EDTA. Thepresence of anionic surfactant (SDS) and nonionic surfactants (Triton X-100) may affect themetal extractive capacity of LED3A because it poses hydrophobic portion (lauryl functiongroup). In this study, the effects of extractants (surfactants, chelating agent, and surfactant-chelating agent mixture) on cadmium and copper removal at different soil solution pH wereinvestigated. Second, sequential extraction analysis was conducted to determine extractivecapacities of extractants to remove metal from different geochemical fractions. Finally,electrolysis was performed to recover metal from soil-washing solution. The objective ofthis study is to assess the feasibility of the integrated soil washing-electrolysis process formetal-contaminated soil remediation.

2. Materials and Methods

2.1 Soil Preparation

The soil was taken from a park on the Chung-Shan Medical University campus. It wassieved (<2 mm), and oven-dried overnight at 105◦C to remove soil moisture. The particlesize distribution was 81.3% sand, 12.1% silt and 6.7% clay. The soil organic matter was2.3% (w/w) (550◦C, 15 min). The soil pH was 6.3 (1 g soil in 2.5 ml distilled water). Thebackground of copper in campus soil was 10.3 mg kg−1. Cadmium concentration was below1 mg kg−1. The cation exchange capacity (CEC) was 18.08 meq/100 g soil. Cd (NO3)2·4H2O(ACROS, Japan) and CuSO4 (Panreac Qulmica, E.U.) were mixed with distilled water toprepare metal solutions. 1.5 ml of metal solution was added into 5 g soil to obtain the desiredsoil metal concentrations (Cd: 20 mg kg−1; Cu: 400 mg kg−1). The contaminated soil wasincubated at 45◦C for 3 days to reduce soil moisture before soil washing.

2.2 Batch Soil Washing

The surfactants and chelating agents selected in this study are Triton X 100 (AmericanBiorganics Inc., USA), SDS (>90% purity, Sigma, USA), Na2EDTA·2H2O (100% purity,Tedia, USA), and LED3A·Na2 (30%, Hampshire Chemical Corp., USA) (Table 1). Toprepare the different extractants, first, two-fold desired concentrations of surfactant (orchelating agent) were prepared with distilled water. To prepare the desired surfactant-alone(or chelating alone) solution, the 20 ml of surfactant solution was mixed with 20 ml ofdistilled water. To obtain desired surfactant-chelating agent solution, 20 ml of 4, 0.4, 0.04,0.004, 0% of surfactants (SDS or Triton X-100) and 20 ml of 0.02, 0.01, 0.002, 0.001, and0 M of LED3A were mixed. The extractant solution was adjusted to desired pH by NaOH

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Table 1Properties of surfactants and chelating agent used in this study

Type Chemical formula MWCMC mM(mg L−1)

Triton X 100 Nonionic surfactant C12H25O(CH2CH2O)4H 647 0.18 (268)1

SDS Anionic surfactant C12H25OSO2ONa 288 8.4 (2420)2

EDTA-Na2 Chelating agent C10H12N2Na2O8·2H2O 372.2 NA3

LED3A-Na2 Anionic surfactant/ Lauroyl-ED3ANa2 459 4 (1700)4

chelating agent

1Edwards et al., 1994.2Deshpande et al., 1999.3Chelating agent, NA = not applicable.4Crudden et al., 1994.

or HNO3 solution. 40 ml of extractant was added into 5 g of metal-contaminated soil in apolyethylene centrifuge tube and mixed on a reciprocating shaker at 30 rpm for 3 hours.

The soil-washing supernatant was removed by centrifugation (1500 rpm, 30 min),filtered by a Whatman 42 filter paper (2.5µm, Whatman, USA), and then analyzed byatomic absorbance spectrophotometry (AAS, Perkin-Elmer, model 3300). Distilled waterwas used to extract the residual extracting solution by following the same procedure, exceptthat the extracting period was shortened to one hour. All experiments were performed intriplicate. Blank control was performed throughout the experiment. For quality control, theartificially contaminated soils were digested in aqua regia and analyzed by AA. Recoveriesof cadmium and copper were in the range 85–115%.

2.3 Sequential Extraction Procedure

The sequential extraction analysis was conducted as described by Tessier et al. (1979),except that the aqua regia digest method was used in the final extraction rather than digestionwith HF-HClO4 mixture. This was because the soil was artificially spiked, and because itis believed that metals in mineral crystal structure are difficult to redistribute and are notlikely to be mobilized.

2.4 Electrolysis Test

All electrolysis tests were conducted with a potentiostat-galvanostat (GW, GPC-3030D,Taiwan). A 400 ml soil-washing solution containing metal (either 4 mg L−1cadmium or50 mg L−1copper), 0.01M LEAD3A and 2% SDS was used for electrolysis tests. Stainlesssteel wool (0.5 mm width, total area: 600 cm2) and stainless plate (3 cm × 8 cm) were usedas anode and cathode, respectively. The current density was maintained at 50A m−2. Theanode was wrapped with nylon net to separate the two electrodes.

3. Results and Discussion

3.1 Effects of Soil Solution pH on Metal Extraction

Concentrations of 2% (w/w) surfactants (SDS: 69 mM; Triton X 100: 31 mM) and0.01 M LED3A were used to evaluate their effects on cadmium (20 mg kg−1) and copper

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(400 mg kg−1) removal. The applied concentrations of 2% surfactants were based on theconcentrations commonly used in soil washing for removal of hydrophobic organic hy-drocarbons (Mulligan et al., 2001). 0.01 M of chelating agent was determined to be theoptimum dosage for maximal metal removal efficiency (data not shown).

The use of water, SDS, or Triton X 100 only had little effect (<6%) on cad-mium removal at pH 5 to 11 (Figure 1a). EDTA removed greater than 80% cadmiumfrom soil. EDTA had a much higher cadmium removal capacity than surfactants (SDS,Triton X100). This is because EDTA has a chelating capacity which can effectively

Figure 1. Influences of pH on metal removals by chelating agent (0.01 M) and surfactants (0.2%).The spiked concentration of cadmium and copper in soil were 20 and 400 mg kg−1, respectively. Eachbar represents the mean and standard deviation. (a) cadmium; (b) copper.

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extract metal from soil, while surfactants do not have any chelating properties. LED3Aremoved cadmium between 30–50% depending on soil solution pH and removal in-creased with higher pH. Although LED3A is structurally similar to EDTA, LED3A hada lower cadmium removal capacity than that of EDTA. It is possible that the hydropho-bic portion of LED3A may sorb onto soil and reduce its cadmium removal capacity.However, the addition of either anionic surfactant (SDS) or nonionic surfactant (TritonX 100) significantly increased the cadmium removal by LED3A (>80%), except forLED3A-SDS when soil solution pH was <7. To our knowledge, this is the first reportto date indicating that surfactants significantly increased the metal removal capacities ofLED3A. The LED3A was much more effective for copper removal than cadmium (Fig-ure 1b); the synergistic effect of the surfactant and LED3A for copper removal was lessobvious.

Anionic surfactants could enhance metal removal from soil through the mechanismsof counterion exchange at micelle surface above CMC, complexation with the surfactantunder CMC, and matrix dissolution (Herman et al., 1995; Nivas et al., 1996; Doong et al.,1998; Mulligan et al., 1999; Gadelle et al., 2001). Unlike anionic surfactant, nonionicsurfactants have no effects on metal removal (Huang et al., 1997). However, in this study,not only anionic surfactants (i.e., SDS) but also nonionic surfactants (i.e., Triton X 100)enhanced the effectiveness of LED3A to remove cadmium and copper. As mentioned above,LED3A possesses acetic acid function groups (hydrophilic) and lauroyl function groups(hydrophobic) (Crudden et al., 1994). The hydrophobic portion of LED3A may sorb ontosoil and reduce its metal removal capacity. The addition of 2% surfactants forms micellesthat could partition the acetic acid function group of the LED3A and avoid the sorption ofLED3A-metal complex on soil, and thus enhances metal desorption. The mechanism willbe further discussed in the next section.

3.2 Effects of Extractant Concentrations

Different concentrations of LED3A, surfactants, and LED3A-surfactant mixture were usedto investigate their effects on cadmium removal. The soil solution pH values were not ad-justed and varied between 7 to 8 following mixing with various concentrations of LED3Aand surfactants. When LED3A was applied alone, cadmium removal increased with theLED3A concentrations, peaking at approximately 40% at 0.01 M LED3A application rate(Figure 2a). The addition of 0.002% SDS retarded cadmium removal by LED3A. It is hy-pothesized that most SDS is sorbed on soil at low SDS concentration. The sorbed SDSprevented the LED3A-cadmium complex being desorbed from soil and reduced the cad-mium removal capacity of LED3A. Edwards et al. (1994) indicated that sediment couldsorb surfactant and reduces the effective surfactant concentration in an aqueous system.Lin and Juang (2002) indicated that SDS-sorbed montmorillonite could effectively removecopper and zinc ions from wastewater.

Figure 2a shows that while the added SDS concentrations increased to above 0.2%, SDSsignificantly enhanced cadmium removal by LED3A.The synergistic effects also increasedwith LED3A concentrations. It is presumed that SDS forms micelles at concentrations of0.2% (w/w) and partitions LED3A-cadmium from soil to aqueous phase, thus increasingcadmium removal. The synergistic effects of LED3A and SDS on cadmium removal be-came more significant when SDS and LED3A concentrations increase to 2% and 0.05 M,respectively (Figure 2a). Similarly, the synergistic effects on cadmium removal increasedwith the nonionic surfactant (i.e., Triton X100) concentrations and LED3A concentrations(Figure 2b). However, unlike anionic surfactant (SDS), Triton X100 at low concentrations

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Figure 2. The effects of LED3A-SDS and LED3A-Triton X 100 mixtures on cadmium removal.The spiked cadmium concentration in soil was 20 mg kg−1. Each point represents the mean andstandard deviation. The pH of soil solution was 7. (a) The mixture of LED3A-SDS; (b) the mixtureof LED3A-Triton X 100.

did not inhibit cadmium removal by LED3A. It is possible that Triton X100 is not likely tointeract with cadmium ions.

Because of the higher copper removal efficiencies of LED3A, only 0.5%, 0.1%, and 2%of surfactants were used to investigate the interactions of surfactants and LED3A on copperremoval (Figure 3). The spiked copper concentration (400 mg kg−1) was much higher thancadmium (20 mg kg−1). Even though LED3A alone had high copper removal efficiencythe synergistic effects by additions of surfactants were still observed. This suggests that

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Figure 3. The effects of LED3A-SDS and LED3A-Triton X 100 mixtures on copper removal. Thespiked copper concentration in soil was 400 mg kg−1. Each point represents the mean and standarddeviation. The pH of soil solution was 7. (a) The mixture of LED3A-SDS; (b) the mixture of LED3A-Triton ×100.

Triton X 100 micelles partition copper-LED3A complex, and thus enhance copper removalby LED3A.

3.3 Sequential Extraction Analysis

After soil washing by different extractants (water, surfactants, LED3A, or surfactant-LED3A mixture), sequential extraction analysis was performed to determine metal

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Figure 4. Results of sequential extraction analysis for cadmium and copper in spiked soil after soilwashing by 2% SDS, 0.01M LED3A, and SDS-LED3A mixture. (a) 20 mg kg−1 cadmium; (b) 400 mgkg−1 copper.

speciation in soil. The results of sequential extraction analysis provided valuable infor-mation on the metal extractive capacity of different extractants. Water removed less than2% cadmium and copper (Figure 4). Although both cadmium and copper were introducedin the soil by spiking with metal inorganic salts, the speciation distribution of copper wassignificantly different from that of cadmium. In the water-washed soil, most of the cad-mium was in the exchange fraction (70%), followed by carbonate (25%), and Fe-Mn oxidefraction (3%). In contrast, most of the copper was in the carbonate fraction (75%), followedby Fe-Mn oxide (22%), and organic fraction (2%).

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Figure 4 shows that 2% of SDS only removed a small amount of cadmium and copper.The addition of 0.01M LED3A alone significantly removed cadmium and copper from soilin all different fractions. Compared to LED3A alone, the additions of SDS into LED3Aremoved further quantities of cadmium and copper from soil. It should be noted that theaddition of LED3A alone removed copper from organic fraction. However, 4–5% copperfrom other fractions was also converted to exchangeable fraction. The addition of SDS intoLED3A not only removed copper from organic fractions but also prevented the conversionof copper into exchangeable fraction.

In sequential extraction analysis, strong oxidants (e.g. hydrogen peroxide) were appliedto determine metal species in the organic fraction (Tessier et al., 1979). In this study,LED3A is capable of removing copper from the organic fraction. LED3A possesses boththe chelating and surface properties. The removal of copper in the organic fraction waspossibly a result of complexation between LED3A rather than dissolution of organic fractioninto LED3A micelles, because the addition of 2% of surfactant SDS showed no effects onremoval of copper in organic fraction (Figure 1b). The phenomenon that chelating agentcould bind with organics (humic substrate), thus enhancing the mobilization of PAHs, wasreported by Yang et al. (2001).

Because of the low toxicity and biodegradability of LED3A, the mixture ofLED3A/surfactant provides promising nontoxic and biodegradable extracting agents formetal removal in metal-contaminated sites or metal-organic co-contaminated soil. Furtherstudy on recovery of heavy metal and extracting agents from extracted solutions is neededto determine its feasibility for field application.

In the historically contaminated soil, higher portions of metals are present in the residualfraction. It is believed that the metals in the residual fraction can only be removed by strongacids, like HF and HNO3. The metals present in the first fractions are more likely to beuptaken by plants. Therefore, results of soil washing for freshly spiked soil still providevaluable information on metal-contaminated soil remediation.

3.4 Electrolysis Treatment

Sequential treatments including alkaline precipitation test and electrolysis test were con-ducted to recover metals from supernatants, which were generated from soil washing pro-cess. The initial cadmium and copper concentrations were 4 mg L−1 and 50 mg L−1, re-spectively. Experimental results showed that the presence of 0.2% SDS and 0.01M LED3Aretarded the precipitation of cadmium and copper at pH 11(<5%, data not shown). There-fore, electrolysis effectively recovered cadmium (93.6%) and copper (99.3%) from super-natant after 30 minutes (Figure 5). This is the first report on recovery of metals and chelatingagent/surfactant from supernatants without membrane separation. Elsherief (2003) reportedthat the spiral stainless steel wool cathode effectively recovers cadmium in dilute sulfatesolution. In this study, the electrolysis recovered both copper and cadmium in the presenceof surfactant (SDS) and chelating agent (LED3A).

The combined soil washing with chelating agent/surfactants and electrolysis providea promising approach to remove and recover copper and cadmium from soils. However,optimal operations such as current efficiency and pH for recovery of cadmium and copperfrom supernatant should be further investigated.

Additionally, because of the high costs of chelating agents and surfactants, pretreatment,involving the separation of the highly metal-contaminated soil from gravel and stones,should be conducted to reduce the volume needed for treatment and, subsequently, the costof extractants. In this study, it has been proved that electrolysis could recover the metals and

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Figure 5. Removal of cadmium and copper from leachate in presence of 0.01 M LED3A and 2%SDS. The initial cadmium and copper concentrations were 4 and 50 mg L−1, respectively, in thesupernatant. Initial pH was 11. Current density was maintained at 50 A m−2.

extractants from the soil-washing supernants. The extractants can be used for subsequent soilwashing. Thus the cost of extractants for soil washing could reduce significantly. However,the chelating agent and surfactants may be lost during soil washing and electrolysis. Furtherinvestigation should be conducted for field application.

4. Conclusions

LED3A was more effective on copper removal than on cadmium. The addition of either SDSor Triton X 100 significantly increased cadmium removal by LED3A. LED3A-surfactantsmixture, excluding LED3A-SDS at pH <7, removed more than 80% cadmium. The en-hancements on cadmium removal are not only related to surfactant concentrations but alsoto LED3A concentrations. However, SDS at concentration of 0.002% inhibited LED3A toremove cadmium.

Sequential extraction analysis indicated that LED3A not only removed copper fromcarbonate and Fe-Mn oxide fraction, but also from organic fractions. Electrolysis at alkalinecondition (pH 11) effectively recovered cadmium and copper from supernatant withoutmembrane separation. The combined soil washing with LED3A-surfactants and electrolysisprovide a potential approach for remediation of cadmium and copper-contaminated soils.

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